In the normal application of a Wheatstone bridge, an applied power voltage is supplied to one power input terminal of the bridge while the other power input terminal is grounded. The portion of the power voltage appearing at the output terminals of the bridge is also known as a common mode voltage. One or more legs of the bridge will generally be formed of electrical transducers, such as variable resistances which vary in response to applied signals, such as signals due to changes in pressure or temperature. The bridge will typically have two values of voltage appearing at the output terminals thereof. At one output terminal, there will be one-half of the power voltage, or the common mode voltage, plus the induced signal voltage due to variations in the transducers of the bridge. At the other output terminal, there will be one-half of the common mode voltage minus the induced signal voltage. The output terminals of the bridge are typically connected to a differential amplifier to measure the induced signal voltage.
The objective of any differential amplifier is to amplify the induced signal voltage without being influenced by the common mode components. If the output terminals of the bridge are applied to the input terminals of two separate amplifiers defining the differential amplifier, the response of those two amplifiers to the relatively large common mode or power voltage components, for example, 5 volts, must be exactly the same so that the common mode components will cancel each other at the outputs of the two amplifiers. If the voltage outputs of the two amplifiers differs slightly, that is, if a gain of one of the amplifiers is, for example, a percent or two different from the gain of the other amplifier, there will then be a contribution to the output voltage of the differential amplifier of 1% or more of the relatively large common mode voltage. This contribution could be 50 millivolts for a common mode voltage of 5 volts.
A typical signal voltage of a Wheatstone bridge is 100 millivolts. If one is concerned with measuring such a signal voltage with the precision of 1 part in 1,000, then one is concerned with a one-tenth of a millivolt. Thus, it is obvious then that a 50 millivolt contribution to the amplifier output that might result due to the unequal amplification by the two amplifiers would be 500 times the error which one is willing to accept. It is important, therefore, that the two amplifiers arranged to cancel this common mode voltage contribution.
In a typical differential amplifier for the output terminals of a Wheatstone bridge are connected to the positive input terminals of a differential amplifier comprised of two amplifiers and a resistance network of three resistances in series across the outputs of the two amplifiers. Return leads are coupled on respective sides of the center resistance and are connected to the negative input terminals of the two amplifiers. In order for the common mode voltage to be totally absent from the output of the differential amplifier, the other two resistances of the network must be exactly the same in resistance value and must remain so with aging, temperature changes, mechanical stresses and other factors which influence their value. This is a difficult requirement to meet and maintain over extended periods of usage of the differential amplifier. Because of this drawback, a need has arisen for an improved differential amplifier which clearly avoids the aforesaid problems yet provides a reliable signal voltage output substantially devoid of any contribution due to a common mode voltage.